1. Field of the Invention
This invention relates to a telephone terminal of a network telephone system for conducting voice communications between telephone terminals through an IP network, such as an IP (Internet Protocol) telephone system, for example, and a power supply method used with the telephone terminal.
2. Description of the Related Art
In recent years, a network telephone system (IP telephone system) for transmitting and receiving an image and voice as packet data bi-directionally in real time through a packet network has begun to become widespread.
In the IP telephone system, inter-extension communications can be conducted and outside line outgoing and incoming calls can be made for each gateway connected to the packet network, of course, and in addition, extension communications can be conducted and outside line outgoing and incoming calls can be made between the gateway via the packet network. In this case, communications can be carried out as power (drive power) is supplied to an IP telephone terminal in accordance with a power supply technique such as POE (Power Over Ethernet), which is described in below.
Power supply is received through the packet network from the gateway.
Power supply is received from a network component of a router, a switching hub, etc., forming a part of a LAN.
Power supply is received from a midspan unit. The midspan unit is a power supply dedicated unit being installed between a “network component with no power supply function” and a “terminal to which power is to be supplied” for allowing a LAN signal to pass through and adding only supply power.
In the IP telephone system, to purchase an IP telephone terminal for use, the user generally selects an IP telephone terminal having necessary functions at the point in time. However, after use of the IP telephone terminal is started, it becomes necessary to add a function in response to change in the use situation, etc. In such a case, a technique for the user of the IP telephone terminal to connect an optional unit for executing any desired function to the IP telephone terminal main unit is proposed. (For example, refer to JP-A-4-192698 and JP-A-4-239898.)